The Public Telephone Access Network
The access network (telephone network) constitutes a star-net-topology, see FIG. 1. The fibre backbone data network is terminated at the Central Office (CO) supporting 500-20000 end customers. From the CO primary cables carrying 100-1200 pairs runs to Cabinets (Cab) which are cross-connect-points. The last 100-800 meters of twisted pairs between the Cab and the customer premises is called the distribution network.
It is desirable to re-use the existing copper access network for delivering high capacity data connections (“broadband”) to the premises. The family of systems designed for this purpose is called DSL (Digital Subscriber Lines) systems.
Statistics of network topology and cable lengths are crucial parameters when deploying DSL. The copper loops have the property that the possible transmission data rates decreases for longer loops because the signals get more attenuated the longer the loop is. A second property that limits the possible data rates is crosstalk, i.e. self made noise, that occurs between different copper pairs in the same cable during transmission.
Common to all currently used DSL systems is that they are designed for a worst case scenario. This means that the systems are designed for a maximum cross-talk scenario, i.e. all systems are transmitting all the time and generates full cross talk to each other. In reality, it is very unlikely that all users are transmitting and receiving data at the same time, and all the time, and thus it can be argued that this optimization criteria would lead to a waste of capacity. An outcome of this is that some users that would need more capacity cannot have it since the system is “saving” capacity in case the maximum cross-talk scenario should occur.
There are two kinds of crosstalk: Near End Cross Talk (NEXT) and Far End Cross Talk (FEXT). NEXT is noise that comes from a transmitter on a neighbouring pair at the same end. FEXT is noise that comes from a transmitter on a neighbouring pair located at the far end of the line. This is illustrated in FIG. 2.
NEXT is always stronger than FEXT and most DSL systems are designed to avoid NEXT but assumes that FEXT always is present. Some systems operating at low frequencies, i.e. frequencies up to 500-800 kHz, are designed to also take NEXT into account. This is possible since NEXT is not very severe at low frequencies (see FIG. 3). At these frequencies the received signal is stronger than the NEXT, and hence there exist a positive signal to noise ratio that can be used for transmission in both directions.
Existing standardized DSL (digital subscriber lines) systems for high capacities (5-10 Mbit/s in both directions) suffers from short reach. VDSL, Very high speed Digital Subscriber Line, is a member in the DSL family that is designed for high capacities (10-52 Mbit/s). The reason why VDSL can offer much higher capacities than, e.g., ADSL, Asymmetric Digital Subscriber Line, and SHDSL, Symmetric High bit rate Digital Subscriber Line, is that the system uses a much larger bandwidth. The VDSL frequency band ranges from some 100 kHz up to 12-20 MHz. ADSL and SHDSL are using frequencies between 0 and maximum 1.1 MHz. Because of the attenuation that increases with frequency and cable length, and the crosstalk, the usable bandwidth decreases very fast for longer loops. This means that the large bandwidth of VDSL is only usable for shorter loops. VDSL can deliver 10 Mbit/s up to 800-1000 meters. For longer loops the attenuation in a VDSL system is too high and the data rates drops quickly down to, or below, the level of ADSL and SHDSL.
The DSL Family
There are several variants of DSL systems standardized in international standardization bodies such as ITU-T (International), T1E1 (North America), and ETSI (Europe). The three most significant systems are briefly presented in the sequel.
ADSL, Asymmetric Digital Subscriber Line
ADSL is the most popular and widely deployed DSL system. The cornerstone for ADSL systems is long reach capability and asymmetric data rates. Asymmetric data rates means that the system is designed for providing a higher data rate in the downstream direction than in the upstream direction. The ADSL network equipment is installed at the central office (local exchange) and operates over the existing copper infrastructure providing services to a majority of existing telephone customers (e.g., 80-90% in European networks). The operator only needs to increase broadband capacity in the backbone and pre-install equipment at the central office in a regional area. He will later connect new customers as they adopt to the service offered in that area. Only small adjustments in existing copper network are required. When using ADSL it is possible to support ordinary telephony (POTS—Plain Ordinary Telephone Service) carried over the same line.
SHDSL, Symmetric High Bit Rate Digital Subscriber Line
SHDSL is a system that is designed to provide symmetric data rates, in the magnitude of 2 Mbit/s in each direction, at long range. It is expected that SHDSL mainly will be used by business customers with the need of LAN interconnect, PABX's (private automatic branch exchange), Internet, etc. When using SHDSL it is not possible to support ordinary analogue telephony (POTS) carried over the same line.
VDSL, Very High Speed Digital Subscriber Line
VDSL is considered to be the next generation broadband technology for the copper networks. It provides higher data bandwidths than ADSL and SHDSL but to the expense of shorter reach. For VDSL, network operators can only partly use the same deployment strategy as for ADSL. From the central office, VDSL can be offered to e.g. 30-50% of customers compared to 80-90% in the case of ADSL. (This depends on the topology of the specific network.) To further increase the VDSL customer base it is necessary to deploy a fibre-to-the-cabinet (FTTCab) infrastructure meaning that the fibre termination point is moved closer to the premises giving a shorter copper loop. The cabinet is deployed at the local cross-connect point for the distribution network, which normally is the only point-of-presence for the cable. (The cabinet is in general the only point where practical operations on the cable are possible.) The VDSL Digital Subscriber Line Access Multiplexer (DSLAM) equipment will be placed in the new cabinet and VDSL is used to serve the customers over the last drops of cable. When using VDSL it is possible to support ordinary telephony carried over the same line.
Deploying new FTTCab infrastructures is a difficult decision since it is considered expensive by many network operators. Dedicated VDSL roll-out investments are done in advance of the market and may lead to uncertain pay-back times for the new cabinets that contain active electronics, broadband equipment, power feeding, and environmental protection. Still there are a number of operators that have plans to do it as a step towards a more modem and data centric access network.
Duplex Methods
The way the available analogue bandwidth is shared in both direction of transmission is described and managed by the use of a duplex method. There are in principle four different duplex methods:
Frequency Divided Duplex (FDD):
In a FDD system the available analogue bandwidth is divided into non-overlapping frequency bands. Each band is used for either up- or downstream transmission. In a FDD system NEXT is avoided, but FEXT will occur. Examples of DSL systems that are based on FDD are ADSL and VDSL.
Time Division Duplex (TDD):
In a TDD system the up- and down stream data is transmitted in different time slots, i.e., the entire analogue bandwidths is used for both up- and downstream transmission, but not at the same time. To avoid NEXT it is necessary that all modems in the same cable is time synchronized, i.e. all modems send upstream data at the same time, and downstream data at the same time. Synchronized TDD is used in Japan for ISDN and a special Japanese variant of ADSL.
Echo Cancelled Duplex (EC), sometimes also denoted as “full duplex”:
Data is transmitted simultaneously in both up- and downstream direction over the same frequency band, i.e., the entire analogue bandwidth is used in both directions. With this technique the modem receiver will receive not only the signal transmitted from the other side of the line, but also its own return echo from the transmitter at the same side. This requires that the modem provides echo cancellation functionality. EC systems suffer from, and are limited by NEXT. SHDSL is a member in the DSL family that utilize EC techniques.
Burst Mode Duplex (BM)
See, for instance, the technology white paper: “Etherloop Spectrum Manager”, Patric Stanley, Elastic Networks, doc. Nr. 08-01063-01. In BM each modem transmits and receives data in up- and downstream in a non synchronized fashion. The modem is silent when it does not have data to send. This means that received data either suffer from NEXT and/or FEXT from other pairs, or not suffers at all from crosstalk. There are no standardized systems based on this duplex method.
Problems With Current And Existing Techniques
ADSL provides asymmetric services at long loops, but is suboptimal when it comes to provide high bandwidth symmetric services at long loops.
VDSL provides high bit rates at short loops, but cannot be used for long loops.
SHDSL provides symmetric long reach services, but the capacity at short loops is much less than VDSL bit rates.
For VDSL systems a technique called Zipper is standardized. Zipper is a time-synchronized frequency division duplex implementation of discrete multi tone (DMT) modulation. The Zipper technique is described in WO 99/43123. A system using the Zipper technique suffers however from the same disadvantage as described for VDSL above, i.e. can not be used for long loops.
In EP 0991202 it is described how echo cancellation can be implemented in a system using Zipper. Here it is described that it is possible to utilize echo cancellation for all carriers in a Zipper system. However, such a system is not optimal for providing high capacities at short loops. Even if the echoes are cancelled for the entire transmission band, the NEXT will drown the high frequency signal and the overall capacity will be lower.